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  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
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  <title>P0750R0: Consume</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in §N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

/******************************************************************************/
/*                                   Body                                     */
/******************************************************************************/

	body {
		counter-reset: example figure issue;

		/* Layout */
		max-width: 50em;               /* limit line length to 50em for readability   */
		margin: 0 auto;                /* center text within page                     */
		padding: 1.6em 1.5em 2em 50px; /* assume 16px font size for downlevel clients */
		padding: 1.6em 1.5em 2em calc(26px + 1.5em); /* leave space for status flag     */

		/* Typography */
		line-height: 1.5;
		font-family: sans-serif;
		widows: 2;
		orphans: 2;
		word-wrap: break-word;
		overflow-wrap: break-word;
		hyphens: auto;

		/* Colors */
		color: black;
		background: white top left fixed no-repeat;
		background-size: 25px auto;
	}


/******************************************************************************/
/*                         Front Matter & Navigation                          */
/******************************************************************************/

/** Header ********************************************************************/

	div.head { margin-bottom: 1em }
	div.head hr { border-style: solid; }

	div.head h1 {
		font-weight: bold;
		margin: 0 0 .1em;
		font-size: 220%;
	}

	div.head h2 { margin-bottom: 1.5em;}

/** W3C Logo ******************************************************************/

	.head .logo {
		float: right;
		margin: 0.4rem 0 0.2rem .4rem;
	}

	.head img[src*="logos/W3C"] {
		display: block;
		border: solid #1a5e9a;
		border-width: .65rem .7rem .6rem;
		border-radius: .4rem;
		background: #1a5e9a;
		color: white;
		font-weight: bold;
	}

	.head a:hover > img[src*="logos/W3C"],
	.head a:focus > img[src*="logos/W3C"] {
		opacity: .8;
	}

	.head a:active > img[src*="logos/W3C"] {
		background: #c00;
		border-color: #c00;
	}

	/* see also additional rules in Link Styling section */

/** Copyright *****************************************************************/

	p.copyright,
	p.copyright small { font-size: small }

/** Back to Top / ToC Toggle **************************************************/

	@media print {
		#toc-nav {
			display: none;
		}
	}
	@media not print {
		#toc-nav {
			position: fixed;
			z-index: 2;
			bottom: 0; left: 0;
			margin: 0;
			min-width: 1.33em;
			border-top-right-radius: 2rem;
			box-shadow: 0 0 2px;
			font-size: 1.5em;
			color: black;
		}
		#toc-nav > a {
			display: block;
			white-space: nowrap;

			height: 1.33em;
			padding: .1em 0.3em;
			margin: 0;

			background: white;
			box-shadow: 0 0 2px;
			border: none;
			border-top-right-radius: 1.33em;
			background: white;
		}
		#toc-nav > #toc-jump {
			padding-bottom: 2em;
			margin-bottom: -1.9em;
		}

		#toc-nav > a:hover,
		#toc-nav > a:focus {
			background: #f8f8f8;
		}
		#toc-nav > a:not(:hover):not(:focus) {
			color: #707070;
		}

		/* statusbar gets in the way on keyboard focus; remove once browsers fix */
		#toc-nav > a[href="#toc"]:not(:hover):focus:last-child {
			padding-bottom: 1.5rem;
		}

		#toc-nav:not(:hover) > a:not(:focus) > span + span {
			/* Ideally this uses :focus-within on #toc-nav */
			display: none;
		}
		#toc-nav > a > span + span {
			padding-right: 0.2em;
		}

		#toc-toggle-inline {
			vertical-align: 0.05em;
			font-size: 80%;
			color: gray;
			color: hsla(203,20%,40%,.7);
			border-style: none;
			background: transparent;
			position: relative;
		}
		#toc-toggle-inline:hover:not(:active),
		#toc-toggle-inline:focus:not(:active) {
			text-shadow: 1px 1px silver;
			top: -1px;
			left: -1px;
		}

		#toc-nav :active {
			color: #C00;
		}
	}

/** ToC Sidebar ***************************************************************/

	/* Floating sidebar */
	@media screen {
		body.toc-sidebar #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			max-width: 80%;
			max-width: calc(100% - 2em - 26px);
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
		}
		body.toc-sidebar #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
		}
		body.toc-sidebar #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	/* Hide main scroller when only the ToC is visible anyway */
	@media screen and (max-width: 28em) {
		body.toc-sidebar {
			overflow: hidden;
		}
	}

	/* Sidebar with its own space */
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
		}
		body:not(.toc-inline) #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
		}

		body:not(.toc-inline) {
			padding-left: 29em;
		}
		/* See also Overflow section at the bottom */

		body:not(.toc-inline) #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) {
			margin: 0 4em;
		}
	}

/******************************************************************************/
/*                                Sectioning                                  */
/******************************************************************************/

/** Headings ******************************************************************/

	h1, h2, h3, h4, h5, h6, dt {
		page-break-after: avoid;
		page-break-inside: avoid;
		font: 100% sans-serif;   /* Reset all font styling to clear out UA styles */
		font-family: inherit;    /* Inherit the font family. */
		line-height: 1.2;        /* Keep wrapped headings compact */
		hyphens: manual;         /* Hyphenated headings look weird */
	}

	h2, h3, h4, h5, h6 {
		margin-top: 3rem;
	}

	h1, h2, h3 {
		color: #005A9C;
		background: transparent;
	}

	h1 { font-size: 170%; }
	h2 { font-size: 140%; }
	h3 { font-size: 120%; }
	h4 { font-weight: bold; }
	h5 { font-style: italic; }
	h6 { font-variant: small-caps; }
	dt { font-weight: bold; }

/** Subheadings ***************************************************************/

	h1 + h2,
	#subtitle {
		/* #subtitle is a subtitle in an H2 under the H1 */
		margin-top: 0;
	}
	h2 + h3,
	h3 + h4,
	h4 + h5,
	h5 + h6 {
		margin-top: 1.2em; /* = 1 x line-height */
	}

/** Section divider ***********************************************************/

	:not(.head) > hr {
		font-size: 1.5em;
		text-align: center;
		margin: 1em auto;
		height: auto;
		border: transparent solid 0;
		background: transparent;
	}
	:not(.head) > hr::before {
		content: "\2727\2003\2003\2727\2003\2003\2727";
	}

/******************************************************************************/
/*                            Paragraphs and Lists                            */
/******************************************************************************/

	p {
		margin: 1em 0;
	}

	dd > p:first-child,
	li > p:first-child {
		margin-top: 0;
	}

	ul, ol {
		margin-left: 0;
		padding-left: 2em;
	}

	li {
		margin: 0.25em 0 0.5em;
		padding: 0;
	}

	dl dd {
		margin: 0 0 .5em 2em;
	}

	.head dd + dd { /* compact for header */
		margin-top: -.5em;
	}

	/* Style for algorithms */
	ol.algorithm ol:not(.algorithm),
	.algorithm > ol ol:not(.algorithm) {
	 border-left: 0.5em solid #DEF;
	}

	/* Put nice boxes around each algorithm. */
	[data-algorithm]:not(.heading) {
	  padding: .5em;
	  border: thin solid #ddd; border-radius: .5em;
	  margin: .5em calc(-0.5em - 1px);
	}
	[data-algorithm]:not(.heading) > :first-child {
	  margin-top: 0;
	}
	[data-algorithm]:not(.heading) > :last-child {
	  margin-bottom: 0;
	}

	/* Style for switch/case <dl>s */
	dl.switch > dd > ol.only,
	dl.switch > dd > .only > ol {
	 margin-left: 0;
	}
	dl.switch > dd > ol.algorithm,
	dl.switch > dd > .algorithm > ol {
	 margin-left: -2em;
	}
	dl.switch {
	 padding-left: 2em;
	}
	dl.switch > dt {
	 text-indent: -1.5em;
	 margin-top: 1em;
	}
	dl.switch > dt + dt {
	 margin-top: 0;
	}
	dl.switch > dt::before {
	 content: '\21AA';
	 padding: 0 0.5em 0 0;
	 display: inline-block;
	 width: 1em;
	 text-align: right;
	 line-height: 0.5em;
	}

/** Terminology Markup ********************************************************/


/******************************************************************************/
/*                                 Inline Markup                              */
/******************************************************************************/

/** Terminology Markup ********************************************************/
	dfn   { /* Defining instance */
		font-weight: bolder;
	}
	a > i { /* Instance of term */
		font-style: normal;
	}
	dt dfn code, code.idl {
		font-size: medium;
	}
	dfn var {
		font-style: normal;
	}

/** Change Marking ************************************************************/

	del { color: red;  text-decoration: line-through; }
	ins { color: #080; text-decoration: underline;    }

/** Miscellaneous improvements to inline formatting ***************************/

	sup {
		vertical-align: super;
		font-size: 80%
	}

/******************************************************************************/
/*                                    Code                                    */
/******************************************************************************/

/** General monospace/pre rules ***********************************************/

	pre, code, samp {
		font-family: Menlo, Consolas, "DejaVu Sans Mono", Monaco, monospace;
		font-size: .9em;
		page-break-inside: avoid;
		hyphens: none;
		text-transform: none;
	}
	pre code,
	code code {
		font-size: 100%;
	}

	pre {
		margin-top: 1em;
		margin-bottom: 1em;
		overflow: auto;
	}

/** Inline Code fragments *****************************************************/

  /* Do something nice. */

/******************************************************************************/
/*                                    Links                                   */
/******************************************************************************/

/** General Hyperlinks ********************************************************/

	/* We hyperlink a lot, so make it less intrusive */
	a[href] {
		color: #034575;
		text-decoration: none;
		border-bottom: 1px solid #707070;
		/* Need a bit of extending for it to look okay */
		padding: 0 1px 0;
		margin: 0 -1px 0;
	}
	a:visited {
		border-bottom-color: #BBB;
	}

	/* Use distinguishing colors when user is interacting with the link */
	a[href]:focus,
	a[href]:hover {
		background: #f8f8f8;
		background: rgba(75%, 75%, 75%, .25);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	a[href]:active {
		color: #C00;
		border-color: #C00;
	}

	/* Backout above styling for W3C logo */
	.head .logo,
	.head .logo a {
		border: none;
		text-decoration: none;
		background: transparent;
	}

/******************************************************************************/
/*                                    Images                                  */
/******************************************************************************/

	img {
		border-style: none;
	}

	/* For autogen numbers, add
	   .caption::before, figcaption::before { content: "Figure " counter(figure) ". "; }
	*/

	figure, .figure, .sidefigure {
		page-break-inside: avoid;
		text-align: center;
		margin: 2.5em 0;
	}
	.figure img,    .sidefigure img,    figure img,
	.figure object, .sidefigure object, figure object {
		max-width: 100%;
		margin: auto;
	}
	.figure pre, .sidefigure pre, figure pre {
		text-align: left;
		display: table;
		margin: 1em auto;
	}
	.figure table, figure table {
		margin: auto;
	}
	@media screen and (min-width: 20em) {
		.sidefigure {
			float: right;
			width: 50%;
			margin: 0 0 0.5em 0.5em
		}
	}
	.caption, figcaption, caption {
		font-style: italic;
		font-size: 90%;
	}
	.caption::before, figcaption::before, figcaption > .marker {
		font-weight: bold;
	}
	.caption, figcaption {
		counter-increment: figure;
	}

	/* DL list is indented 2em, but figure inside it is not */
	dd > .figure, dd > figure { margin-left: -2em }

/******************************************************************************/
/*                             Colored Boxes                                  */
/******************************************************************************/

	.issue, .note, .example, .assertion, .advisement, blockquote {
		padding: .5em;
		border: .5em;
		border-left-style: solid;
		page-break-inside: avoid;
	}
	span.issue, span.note {
		padding: .1em .5em .15em;
		border-right-style: solid;
	}

	.issue,
	.note,
	.example,
	.advisement,
	.assertion,
	blockquote {
		margin: 1em auto;
	}
	.note  > p:first-child,
	.issue > p:first-child,
	blockquote > :first-child {
		margin-top: 0;
	}
	blockquote > :last-child {
		margin-bottom: 0;
	}

/** Blockquotes ***************************************************************/

	blockquote {
		border-color: silver;
	}

/** Open issue ****************************************************************/

	.issue {
		border-color: #E05252;
		background: #FBE9E9;
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		text-transform: uppercase;
		color: #AE1E1E;
		padding-right: 1em;
		text-transform: uppercase;
	}
	/* Add .issue::before { content: "Issue " counter(issue) " "; } for autogen numbers,
	   or use class="marker" to mark up the issue number in source. */

/** Example *******************************************************************/

	.example {
		border-color: #E0CB52;
		background: #FCFAEE;
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		text-transform: uppercase;
		color: #827017;
		min-width: 7.5em;
		display: block;
	}
	/* Add .example::before { content: "Example " counter(example) " "; } for autogen numbers,
	   or use class="marker" to mark up the example number in source. */

/** Non-normative Note ********************************************************/

	.note {
		border-color: #52E052;
		background: #E9FBE9;
		overflow: auto;
	}

	.note::before, .note > .marker,
	details.note > summary::before,
	details.note > summary > .marker {
		text-transform: uppercase;
		display: block;
		color: hsl(120, 70%, 30%);
	}
	/* Add .note::before { content: "Note"; } for autogen label,
	   or use class="marker" to mark up the label in source. */

	details.note > summary {
		display: block;
		color: hsl(120, 70%, 30%);
	}
	details.note[open] > summary {
		border-bottom: 1px silver solid;
	}

/** Assertion Box *************************************************************/
	/*  for assertions in algorithms */

	.assertion {
		border-color: #AAA;
		background: #EEE;
	}

/** Advisement Box ************************************************************/
	/*  for attention-grabbing normative statements */

	.advisement {
		border-color: orange;
		border-style: none solid;
		background: #FFEECC;
	}
	strong.advisement {
		display: block;
		text-align: center;
	}
	.advisement > .marker {
		color: #B35F00;
	}

/** Spec Obsoletion Notice ****************************************************/
	/* obnoxious obsoletion notice for older/abandoned specs. */

	details {
		display: block;
	}
	summary {
		font-weight: bolder;
	}

	.annoying-warning:not(details),
	details.annoying-warning:not([open]) > summary,
	details.annoying-warning[open] {
		background: #fdd;
		color: red;
		font-weight: bold;
		padding: .75em 1em;
		border: thick red;
		border-style: solid;
		border-radius: 1em;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

@media not print {
	details.annoying-warning[open] {
		position: fixed;
		left: 1em;
		right: 1em;
		bottom: 1em;
		z-index: 1000;
	}
}

	details.annoying-warning:not([open]) > summary {
		text-align: center;
	}

/** Entity Definition Boxes ***************************************************/

	.def {
		padding: .5em 1em;
		background: #DEF;
		margin: 1.2em 0;
		border-left: 0.5em solid #8CCBF2;
	}

/******************************************************************************/
/*                                    Tables                                  */
/******************************************************************************/

	th, td {
		text-align: left;
		text-align: start;
	}

/** Property/Descriptor Definition Tables *************************************/

	table.def {
		/* inherits .def box styling, see above */
		width: 100%;
		border-spacing: 0;
	}

	table.def td,
	table.def th {
		padding: 0.5em;
		vertical-align: baseline;
		border-bottom: 1px solid #bbd7e9;
	}

	table.def > tbody > tr:last-child th,
	table.def > tbody > tr:last-child td {
		border-bottom: 0;
	}

	table.def th {
		font-style: italic;
		font-weight: normal;
		padding-left: 1em;
		width: 3em;
	}

	/* For when values are extra-complex and need formatting for readability */
	table td.pre {
		white-space: pre-wrap;
	}

	/* A footnote at the bottom of a def table */
	table.def           td.footnote {
		padding-top: 0.6em;
	}
	table.def           td.footnote::before {
		content: " ";
		display: block;
		height: 0.6em;
		width: 4em;
		border-top: thin solid;
	}

/** Data tables (and properly marked-up index tables) *************************/
	/*
		 <table class="data"> highlights structural relationships in a table
		 when correct markup is used (e.g. thead/tbody, th vs. td, scope attribute)

		 Use class="complex data" for particularly complicated tables --
		 (This will draw more lines: busier, but clearer.)

		 Use class="long" on table cells with paragraph-like contents
		 (This will adjust text alignment accordingly.)
		 Alternately use class="longlastcol" on tables, to have the last column assume "long".
	*/

	table {
		word-wrap: normal;
		overflow-wrap: normal;
		hyphens: manual;
	}

	table.data,
	table.index {
		margin: 1em auto;
		border-collapse: collapse;
		border: hidden;
		width: 100%;
	}
	table.data caption,
	table.index caption {
		max-width: 50em;
		margin: 0 auto 1em;
	}

	table.data td,  table.data th,
	table.index td, table.index th {
		padding: 0.5em 1em;
		border-width: 1px;
		border-color: silver;
		border-top-style: solid;
	}

	table.data thead td:empty {
		padding: 0;
		border: 0;
	}

	table.data  thead,
	table.index thead,
	table.data  tbody,
	table.index tbody {
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/*
Alternate table alignment rules

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Possible extra rowspan handling

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		border-right: 1px solid silver;
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*/

/******************************************************************************/
/*                                  Indices                                   */
/******************************************************************************/


/** Table of Contents *********************************************************/

	.toc a {
		/* More spacing; use padding to make it part of the click target. */
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		#toc li li li li .content {
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/** Index *********************************************************************/

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/******************************************************************************/
/*                                    Legacy                                  */
/******************************************************************************/

	/* This rule is inherited from past style sheets. No idea what it's for. */
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/******************************************************************************/
/*                             Overflow Control                               */
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 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P0750R0<br>Consume</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="1970-01-01">1970-01-01</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/P0750">http://wg21.link/P0750</a>
     <dt>Author:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:jfbastien@apple.com">JF Bastien</a> (<span class="p-org org">Apple</span>)
     <dt>Audience:
     <dd>SG1
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
     <dt>Source:
     <dd><a href="https://github.com/jfbastien/papers/blob/master/source/P0750r0.bs">github.com/jfbastien/papers/blob/master/source/P0750r0.bs</a>
     <dt>Implementation:
     <dd><a href="https://github.com/jfbastien/stdconsume">github.com/jfbastien/stdconsume</a>
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>Fixing memory order consume.</p>
  </div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li><a href="#bg"><span class="secno">1</span> <span class="content">Background</span></a>
    <li><a href="#wat"><span class="secno">2</span> <span class="content">What is <code class="highlight"><c- n>consume</c-></code>?</span></a>
    <li><a href="#old-word"><span class="secno">3</span> <span class="content">Wording as of C++17</span></a>
    <li><a href="#other"><span class="secno">4</span> <span class="content">Other considerations</span></a>
    <li><a href="#future"><span class="secno">5</span> <span class="content">Future work</span></a>
    <li><a href="#api"><span class="secno">6</span> <span class="content">Proposed API</span></a>
    <li><a href="#ack"><span class="secno">7</span> <span class="content">Acknowledgements</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="bg"><span class="secno">1. </span><span class="content">Background</span><a class="self-link" href="#bg"></a></h2>
   <p>Memory order consume as specified in C++11 hasn’t been implemented by any
compiler. The main reason for this shortcoming is that the specification creates
a fundamental mismatch between the model of "dependency" at the C++ source code
level and "dependency" at the ISA-specific assembly level.</p>
   <p>Programmers targeting large systems have been using "consume" under different
names since at least the mid-90’s. As hardware evolved, what used to be an
esoteric memory detail of large systems has become commonplace. For example,
Linux gained support for Read Copy Update <a data-link-type="biblio" href="#biblio-rcu">[RCU]</a> in October 2002 and usage has
steadily increased since then <a data-link-type="biblio" href="#biblio-rcuusage">[RCUUsage]</a>. Memory order consume is the key to
RCU’s scalability.</p>
   <p>This proposal continues work from:</p>
   <ol>
    <li data-md>
     <p>Towards Implementation and Use of <code class="highlight"><c- n>memory</c-> <c- n>order</c-> <c- n>consume</c-></code> <a data-link-type="biblio" href="#biblio-p0098r1">[p0098R1]</a> lists
 a plethora of manners in which C++ could expose consume. In particular,
 section 7.11 is the root of the current proposal.</p>
    <li data-md>
     <p>Marking memory order consume Dependency Chains <a data-link-type="biblio" href="#biblio-p0462r1">[p0462r1]</a> expands on the
 previous paper. Of particular interest is section 3.2 which proposes <code class="highlight"><c- n>depending_ptr</c-></code>, which the current proposal’s API derives from.</p>
    <li data-md>
     <p>Proposal for New memory order consume Definition <a data-link-type="biblio" href="#biblio-p0190r3">[p0190r3]</a> has
 comprehensive details on each operation which RCU supports. That list was
 critical in crafting the precise semantics which the current proposal has.</p>
   </ol>
   <p>The current proposal is different from prior proposals in that it contains a
single proposed API, which is known to work for at least one code base. Indeed,
it is based on work in WebKit <a data-link-type="biblio" href="#biblio-atomics">[Atomics]</a> which has been successfully using
consume memory ordering in C++ on ARM platforms for over a year.</p>
   <p>The WebKit API was written by low-level compiler experts for their own use, with
limited applicability compared to RCU, and isn’t necessarily user-friendly. The
current proposal therefore differs as follows:</p>
   <ul>
    <li data-md>
     <p>Fewer sharp edges than the WebKit one; and</p>
    <li data-md>
     <p>Attempt to support more usecases as documented by <a data-link-type="biblio" href="#biblio-p0462r1">[p0462r1]</a></p>
   </ul>
   <h2 class="heading settled" data-level="2" id="wat"><span class="secno">2. </span><span class="content">What is <code class="highlight"><c- n>consume</c-></code>?</span><a class="self-link" href="#wat"></a></h2>
   <p>Consume exposes low-level architecture dependencies as available in weak memory
multicore systems. For example, it maps to what ARM calls the "address
dependency rule" as explained in section 6.3 of the ARM Barrier Litmus Tests and
Cookbook <a data-link-type="biblio" href="#biblio-litmus">[Litmus]</a>. POWER has a similar ISA feature. On weak memory ISAs,
consume allows writing release / acquire style code, where the reader side
doesn’t need extra fences. Ordering is guaranteed by creating dependencies on
the reader side between the released location and the subsequent reads that
should be observed to happen after that location’s value was written by the
writer. A "dependency" at the ISA level means that computation of the value of
the address of subsequent loads depend on the value loaded from the release
location.</p>
   <p>It is always valid for an implementation to "promote" consume to acquire, but in
most cases this adds an extra fence.</p>
   <p>Despite being used extensively in multiple systems, crafting a consume API which
is easy to use correctly remains a difficult task because it requires very
precise assembly generation. To maintain correctness, current uses rely on:</p>
   <ul>
    <li data-md>
     <p>A gentleperson’s agreement with compiler authors to avoid breaking code;</p>
    <li data-md>
     <p>Human inspection of compiled code disassembly;</p>
    <li data-md>
     <p>Extensive testing;</p>
    <li data-md>
     <p>Judicious application of inline assembly; and</p>
    <li data-md>
     <p>Luck 🍀.</p>
   </ul>
   <p>This proposal is no different, except that it offloads this burden on the
compiler and standard library authors. These people are in a much better
position to ensure correctness: they rely on more engineering than luck to
maintain the language’s correctness because it’s literally the main purpose of
their software.</p>
   <h2 class="heading settled" data-level="3" id="old-word"><span class="secno">3. </span><span class="content">Wording as of C++17</span><a class="self-link" href="#old-word"></a></h2>
   <blockquote>
     ❡7 An evaluation A carries a dependency to an evaluation B if 
    <ol>
     <li>
      the value of A is used as an operand of B, unless: 
      <ol>
       <li>B is an invocation of any specialization of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>kill_dependency</c-></code>, or 
       <li>A is the left operand of a built-in logical AND <code class="highlight"><c- o>&amp;&amp;</c-></code> or logical OR <code class="highlight"><c- o>||</c-></code> operator, or 
       <li>A is the left operand of a conditional <code class="highlight"><c- o>?:</c-></code> operator, or 
       <li>A is the left operand of the built-in comma <code class="highlight"><c- p>,</c-></code> operator; or 
      </ol>
     <li>A writes a scalar object or bit-field M, B reads the value written by A from M, and A is sequenced before B, or 
     <li>for some evaluation X, A carries a dependency to X, and X carries a dependency to B. 
    </ol>
    <p>[ Note: “Carries a dependency to” is a subset of “is sequenced before”, and
      is similarly strictly intra-thread. — end note ]</p>
    <p>❡8 An evaluation A is dependency-ordered before an evaluation B if</p>
    <ol>
     <li>A performs a release operation on an atomic object M, and, in another
        thread, B performs a consume operation on M and reads a value written
        by any side effect in the release sequence headed by A, or 
     <li>for some evaluation X, A is dependency-ordered before X and X carries
        a dependency to B. 
    </ol>
    <p>[ Note: The relation “is dependency-ordered before” is analogous to
      “synchronizes with”, but uses release/consume in place of release/acquire.
      — end note ]</p>
   </blockquote>
   <p>The above wording and subsequent uses of it terms need to be updated.</p>
   <h2 class="heading settled" data-level="4" id="other"><span class="secno">4. </span><span class="content">Other considerations</span><a class="self-link" href="#other"></a></h2>
   <p>The following can also be considered as part of this proposal:</p>
   <ol>
    <li data-md>
     <p>Deprecate passing <code class="highlight"><c- n>memory_order_consume</c-></code> to the existing variable functions.</p>
    <li data-md>
     <p>Make atomic ordering a template parameter tag instead of a variable. This
 would be a huge change, maybe we should do it separately, but it would
 simplify this proposal if we had it.</p>
    <li data-md>
     <p>Deprecate attribute <code class="highlight"><c- p>[[</c-><c- n>carries_dependency</c-><c- p>]]</c-></code>.</p>
    <li data-md>
     <p>C compatibility.</p>
   </ol>
   <p>The author is happy to add these to the current proposal if SG1 thinks it would
be useful to explore.</p>
   <h2 class="heading settled" data-level="5" id="future"><span class="secno">5. </span><span class="content">Future work</span><a class="self-link" href="#future"></a></h2>
   <p>The current proposal has a rough API, very rough wording, and a small number of
tests. More work needs to go towards these items. Furthermore, the following
operations need to be added:</p>
   <ul>
    <li data-md>
     <p>xchg</p>
    <li data-md>
     <p>cmpxchg</p>
    <li data-md>
     <p>read-modify-write</p>
    <li data-md>
     <p>dependent member pointers, returning <code class="highlight"><c- o>&amp;</c-><c- p>(</c-><c- n>ptr1</c-><c- o>->*</c-><c- n>ptr2</c-><c- p>)</c-></code></p>
   </ul>
   <p>It seems like the easiest way to go about is to try out the API in a
from-scratch C++ codebase. An obvious candidate would be a user-space RCU
implementation in C++ using this API.</p>
   <p>It is also worth considering whether consumable types should be restricted
somehow. <code class="highlight"><c- n>atomic</c-></code> requires trivially-copyable types, but doesn’t restrict the
size of <code class="highlight"><c- n>T</c-></code>. Should consume require the same restriction? Is it useful to
consume very large values? In the author’s experience it isn’t, but it could be
useful in other fields, and at a minimum what qualifies as "large" is
ISA-dependent.</p>
   <p>Generated assembly needs to be inspected on multiple implementations to make
sure various compilers and ISAs can support the proposed design. Already the
sample implementation has been observed spilling dependencies to the stack on
ARM64 using GCC, which happens to be valid but might be brittle.</p>
   <h2 class="heading settled" data-level="6" id="api"><span class="secno">6. </span><span class="content">Proposed API</span><a class="self-link" href="#api"></a></h2>
   <p>The following is a proposed API, a full implementation and tests are available
on GitHub under <a data-link-type="biblio" href="#biblio-stdconsume">[StdConsume]</a>.</p>
<pre class="highlight"><c- k>class</c-> <c- nc>dependency</c-><c- p>;</c->

<c- c1>// A value and its dependency.</c->
<c- c1>// FIXME: should this be opaque, to allow T to carry the dependency implicitly?</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- p>,</c-> <c- k>typename</c-> <c- n>Dependency</c-> <c- o>=</c-> <c- n>dependency</c-><c- o>></c->
<c- k>struct</c-> <c- n>dependent</c-> <c- p>{</c->
    <c- n>T</c-> <c- n>value</c-><c- p>;</c->
    <c- n>Dependency</c-> <c- n>dependency</c-><c- p>;</c->

    <c- n>dependent</c-><c- p>()</c-> <c- o>=</c-> <c- k>delete</c-><c- p>;</c->
    <c- n>dependent</c-><c- p>(</c-><c- n>T</c-> <c- n>value</c-><c- p>,</c-> <c- n>Dependency</c-> <c- n>dependency</c-><c- p>)</c-> <c- o>:</c-> <c- n>value</c-><c- p>(</c-><c- n>value</c-><c- p>),</c-> <c- n>dependency</c-><c- p>(</c-><c- n>dependency</c-><c- p>)</c-> <c- p>{}</c->
    <c- n>dependent</c-><c- p>(</c-><c- n>T</c-> <c- n>value</c-><c- p>)</c-> <c- o>:</c-> <c- n>value</c-><c- p>(</c-><c- n>value</c-><c- p>),</c-> <c- n>dependency</c-><c- p>(</c-><c- n>value</c-><c- p>)</c-> <c- p>{}</c->
<c- p>};</c->

<c- c1>// A dependent_ptr contains a pointer which was obtained through a consume load</c->
<c- c1>// operation. It supports a restricted set of operations compared to regular</c->
<c- c1>// pointers, which allows it to continue carrying its dependency.</c->
<c- c1>//</c->
<c- c1>// dependent_ptr differs from dependent&lt;T*> by having a single data member, and</c->
<c- c1>// by acting similarly to how regular pointers act. A dependent_ptr is a useful</c->
<c- c1>// abstraction because it closely matches the low-level details of modern</c->
<c- c1>// ISA-specific dependencies.</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c->
<c- k>class</c-> <c- nc>dependent_ptr</c-> <c- p>{</c->
<c- k>public</c-><c- o>:</c->
    <c- c1>// Constructors</c->

    <c- c1>// No dependency yet.</c->
    <c- n>dependent_ptr</c-><c- p>()</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
    <c- n>dependent_ptr</c-><c- p>(</c-><c- n>T</c-><c- o>*</c-><c- p>);</c-> <c- c1>// FIXME: should this automatically create a dependency as well?</c->
    <c- n>dependent_ptr</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>nullptr_t</c-><c- p>);</c->

    <c- c1>// With dependency.</c->
    <c- n>dependent_ptr</c-><c- p>(</c-><c- n>T</c-><c- o>*</c-><c- p>,</c-> <c- n>dependency</c-><c- p>);</c->
    <c- n>dependent_ptr</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>nullptr_t</c-><c- p>,</c-> <c- n>dependency</c-><c- p>);</c->
    <c- n>dependent_ptr</c-><c- p>(</c-><c- n>dependent</c-><c- o>&lt;</c-><c- b>uintptr_t</c-><c- o>></c-><c- p>);</c->
    <c- n>dependent_ptr</c-><c- p>(</c-><c- n>dependent</c-><c- o>&lt;</c-><c- b>intptr_t</c-><c- o>></c-><c- p>);</c->

    <c- c1>// Copy construction extends the right-hand side chain to cover both</c->
    <c- c1>// dependent pointers. The left-hand side chain is broken.</c->
    <c- n>dependent_ptr</c-><c- p>(</c-><c- k>const</c-> <c- n>dependent_ptr</c-><c- o>&amp;</c-><c- p>);</c->

    <c- c1>// Moving, Copying, and Casting</c->

    <c- c1>// Assigning a non-dependent right-hand side breaks the left-hand side’s</c->
    <c- c1>// chain.</c->
    <c- n>dependent_ptr</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- n>T</c-><c- o>*</c-><c- p>);</c->
    <c- n>dependent_ptr</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>nullptr_t</c-><c- p>);</c->

    <c- c1>// Using a dependent pointer as the right-hand side of an assignment</c->
    <c- c1>// expression extends the chain to cover both the assignment and the value</c->
    <c- c1>// returned by that assignment statement.</c->
    <c- n>dependent_ptr</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- k>const</c-> <c- n>dependent_ptr</c-><c- o>&amp;</c-><c- p>);</c->

    <c- c1>// If a pointer value is part of a dependency chain, then converting it to</c->
    <c- c1>// intptr_t or uintptr_t extends the chain to the result’s dependency. This</c->
    <c- c1>// can be used to perform pointer tagging (with usual C++ caveats on pointer</c->
    <c- c1>// tagging) while retaining dependencies.</c->
    <c- n>dependent</c-><c- o>&lt;</c-><c- b>uintptr_t</c-><c- o>></c-> <c- n>to_uintptr_t</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->
    <c- n>dependent</c-><c- o>&lt;</c-><c- b>intptr_t</c-><c- o>></c-> <c- n>to_intptr_t</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->

    <c- c1>// Pointer Offsets</c->

    <c- c1>// Indexing though a dependent pointer extends the chain to the resulting</c->
    <c- c1>// value.</c->
    <c- n>dependent</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- k>operator</c-><c- p>[](</c-><c- b>size_t</c-> <c- n>offset</c-><c- p>)</c-> <c- k>const</c-><c- p>;</c->

    <c- c1>// Class-member access operators can be thought of as computing an offset. The</c->
    <c- c1>// access itself is in the dependency chain, but such access does not extend</c->
    <c- c1>// the chain to cover the result.</c->
    <c- n>T</c-><c- o>*</c-> <c- k>operator</c-><c- o>-></c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->

    <c- c1>// Dereferencing and Address-Of</c->

    <c- c1>// Dereferencing a dependent pointer extends the chain to the resulting value.</c->
    <c- n>dependent</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- k>operator</c-><c- o>*</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->

    <c- c1>// If a pointer is part of a dependency chain, then applying the unary &amp;</c->
    <c- c1>// address-of operator extends the chain to the result.</c->
    <c- n>dependent_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>*></c-> <c- k>operator</c-><c- o>&amp;</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->

    <c- c1>// In some circumstances, such as for function pointers, the raw pointer</c->
    <c- c1>// value is required. The chain extend to that value.</c->
    <c- n>T</c-><c- o>*</c-> <c- nf>value</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->

    <c- c1>// A pure dependency from the dependent_ptr.</c->
    <c- n>dependency</c-> <c- nf>dependency</c-><c- p>()</c-> <c- k>const</c-><c- p>;</c->

    <c- c1>// Comparisons aren’t needed because the T* themselves can be compared</c->
    <c- c1>// without breaking the dependency chain of the dependent_ptr. This is</c->
    <c- c1>// important because compilers could optimize certain accesses based on the</c->
    <c- c1>// result of comparisons, breaking explicitly constructed chains in the</c->
    <c- c1>// process.</c->

<c- k>private</c-><c- o>:</c->
    <c- c1>// Exposition only:</c->
    <c- n>T</c-><c- o>*</c-> <c- n>ptr</c-> <c- p>{</c-> <c- k>nullptr</c-> <c- p>};</c->
<c- p>};</c->

<c- c1>// A dependency is an opaque value which can be chained through consume</c->
<c- c1>// operations. Chaining dependencies ensures that load operations carry</c->
<c- c1>// dependencies between each other. Dependencies can also be combined to create</c->
<c- c1>// a new dependency which implies a dependency on the combined inputs.</c->
<c- c1>//</c->
<c- c1>// [ Note: Dependencies create false dependencies as defined by existing ISAs.</c->
<c- c1>//   — end note ]</c->
<c- k>class</c-> <c- nc>dependency</c-> <c- p>{</c->
<c- k>public</c-><c- o>:</c->
    <c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-> <c- n>dependency</c-><c- p>(</c-><c- n>T</c-><c- p>);</c->
    <c- n>dependency</c-><c- p>()</c-> <c- o>=</c-> <c- k>delete</c-><c- p>;</c->

    <c- c1>// Dependency combination.</c->
    <c- n>dependency</c-> <c- k>operator</c-><c- o>|</c-><c- p>(</c-><c- n>dependency</c-> <c- n>d</c-><c- p>);</c->
    <c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-> <c- k>friend</c-> <c- n>dependency</c-> <c- k>operator</c-><c- o>|</c-><c- p>(</c-><c- n>dependency</c-><c- p>,</c-> <c- n>dependent_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>);</c->
    <c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-> <c- k>friend</c-> <c- n>dependency</c-> <c- k>operator</c-><c- o>|</c-><c- p>(</c-><c- n>dependent_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>,</c-> <c- n>dependency</c-><c- p>);</c->

    <c- c1>// Pointer tagging.</c->
    <c- k>friend</c-> <c- b>uintptr_t</c-> <c- k>operator</c-><c- o>|</c-><c- p>(</c-><c- n>dependency</c-><c- p>,</c-> <c- b>uintptr_t</c-><c- p>);</c->
    <c- k>friend</c-> <c- b>uintptr_t</c-> <c- k>operator</c-><c- o>|</c-><c- p>(</c-><c- b>uintptr_t</c-><c- p>,</c-> <c- n>dependency</c-><c- p>);</c->
    <c- k>friend</c-> <c- b>intptr_t</c-> <c- k>operator</c-><c- o>|</c-><c- p>(</c-><c- n>dependency</c-><c- p>,</c-> <c- b>intptr_t</c-><c- p>);</c->
    <c- k>friend</c-> <c- b>intptr_t</c-> <c- k>operator</c-><c- o>|</c-><c- p>(</c-><c- b>intptr_t</c-><c- p>,</c-> <c- n>dependency</c-><c- p>);</c->

    <c- c1>// Exposition only:</c->
    <c- k>typedef</c-> <c- b>unsigned</c-> <c- n>dependency_type</c-><c- p>;</c->

<c- k>private</c-><c- o>:</c->
    <c- c1>// Exposition only:</c->
    <c- n>dependency_type</c-> <c- n>dep</c-><c- p>;</c->
<c- p>};</c->

<c- c1>// Free-function dependency combination.</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-> <c- n>dependency</c-> <c- k>operator</c-><c- o>|</c-><c- p>(</c-><c- n>dependency</c-><c- p>,</c-> <c- n>dependent_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>);</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-> <c- n>dependency</c-> <c- k>operator</c-><c- o>|</c-><c- p>(</c-><c- n>dependent_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>,</c-> <c- n>dependency</c-><c- p>);</c->

<c- c1>// Free-function pointer tagging with dependency.</c->
<c- b>uintptr_t</c-> <c- k>operator</c-><c- o>|</c-><c- p>(</c-><c- n>dependency</c-><c- p>,</c-> <c- b>uintptr_t</c-><c- p>);</c->
<c- b>uintptr_t</c-> <c- k>operator</c-><c- o>|</c-><c- p>(</c-><c- b>uintptr_t</c-><c- p>,</c-> <c- n>dependency</c-><c- p>);</c->
<c- b>intptr_t</c-> <c- k>operator</c-><c- o>|</c-><c- p>(</c-><c- n>dependency</c-><c- p>,</c-> <c- b>intptr_t</c-><c- p>);</c->
<c- b>intptr_t</c-> <c- k>operator</c-><c- o>|</c-><c- p>(</c-><c- b>intptr_t</c-><c- p>,</c-> <c- n>dependency</c-><c- p>);</c->

<c- c1>// Beginning of dependency chain.</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-> <c- n>dependent_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- n>consume_load</c-><c- p>(</c-><c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>*>&amp;</c-><c- p>);</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-> <c- n>dependent</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- n>consume_load</c-><c- p>(</c-><c- k>const</c-> <c- n>std</c-><c- o>::</c-><c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>&amp;</c-><c- p>);</c->

<c- c1>// Subsequent dependent operations.</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-> <c- n>dependent_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- n>consume_load</c-><c- p>(</c-><c- n>dependent_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>*></c-><c- p>);</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-> <c- n>dependent</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- n>consume_load</c-><c- p>(</c-><c- n>dependent_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>);</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-> <c- n>dependent_ptr</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- n>consume_load</c-><c- p>(</c-><c- n>T</c-><c- o>**</c-><c- p>,</c-> <c- n>dependency</c-><c- p>);</c->
<c- k>template</c-><c- o>&lt;</c-><c- k>typename</c-> <c- n>T</c-><c- o>></c-> <c- n>dependent</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- n>consume_load</c-><c- p>(</c-><c- n>T</c-><c- o>*</c-><c- p>,</c-> <c- n>dependency</c-><c- p>);</c->
</pre>
   <h2 class="heading settled" data-level="7" id="ack"><span class="secno">7. </span><span class="content">Acknowledgements</span><a class="self-link" href="#ack"></a></h2>
   <p>The Committee’s SG1 sub-group has had many lengthy discussions about memory
order consume, and how to fix it. It took many failed attempts to come to this
solution. In particular, the following people were tremendously helpful in
reviewing this paper: Hans Boehm, Olivier Giroux, Paul McKenney, Robin Morisset,
Tim Northover, Will Deacon. Filip Pizło contributed substantially to the WebKit
implementation and usage of consume, including the first valuable usecase which
I saw as a perfect excuse to try out a consume solution that I had in mind.</p>
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  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-atomics">[Atomics]
   <dd>JF Bastien; Filip Jerzy Pizło. <a href="https://trac.webkit.org/browser/webkit/trunk/Source/WTF/wtf/Atomics.h?rev=+217722#L342">WebKit source: WTF Atomics.h</a>. June 2, 2017. URL: <a href="https://trac.webkit.org/browser/webkit/trunk/Source/WTF/wtf/Atomics.h?rev=+217722#L342">https://trac.webkit.org/browser/webkit/trunk/Source/WTF/wtf/Atomics.h?rev=+217722#L342</a>
   <dt id="biblio-litmus">[Litmus]
   <dd>ARM Limited. <a href="http://infocenter.arm.com/help/topic/com.arm.doc.genc007826/Barrier_Litmus_Tests_and_Cookbook_A08.pdf">ARM Barrier Litmus Tests and Cookbook</a>. November 26, 2009. URL: <a href="http://infocenter.arm.com/help/topic/com.arm.doc.genc007826/Barrier_Litmus_Tests_and_Cookbook_A08.pdf">http://infocenter.arm.com/help/topic/com.arm.doc.genc007826/Barrier_Litmus_Tests_and_Cookbook_A08.pdf</a>
   <dt id="biblio-p0098r1">[p0098R1]
   <dd>Paul E. McKenney, Torvald Riegel, Jeff Preshing, Hans Boehm, Clark Nelson, Olivier Giroux, Lawrence Crowl. <a href="https://wg21.link/p0098r1">Towards Implementation and Use of memory order consume</a>. 4 January 2016. URL: <a href="https://wg21.link/p0098r1">https://wg21.link/p0098r1</a>
   <dt id="biblio-p0190r3">[P0190R3]
   <dd>Paul E. McKenney, Michael Wong, Hans Boehm, Jens Maurer, Jeffrey Yasskin, JF Bastien. <a href="https://wg21.link/p0190r3">Proposal for New memory order consume Definition</a>. 5 February 2017. URL: <a href="https://wg21.link/p0190r3">https://wg21.link/p0190r3</a>
   <dt id="biblio-p0462r1">[P0462R1]
   <dd>Paul E. McKenney, Torvald Riegel, Jeff Preshing, Hans Boehm, Clark Nelson, Olivier Giroux, Lawrence Crowl, JF Bastien, Micheal Wong. <a href="https://wg21.link/p0462r1">Marking memory order consume Dependency Chains</a>. 5 February 2017. URL: <a href="https://wg21.link/p0462r1">https://wg21.link/p0462r1</a>
   <dt id="biblio-rcu">[RCU]
   <dd>Paul McKenney. <a href="https://www.kernel.org/doc/Documentation/RCU/whatisRCU.txt">What is RCU?</a>. August 17, 2017. URL: <a href="https://www.kernel.org/doc/Documentation/RCU/whatisRCU.txt">https://www.kernel.org/doc/Documentation/RCU/whatisRCU.txt</a>
   <dt id="biblio-rcuusage">[RCUUsage]
   <dd>Paul McKenney. <a href="http://www.rdrop.com/users/paulmck/RCU/linuxusage.html">RCU Linux Usage</a>. July 2, 2017. URL: <a href="http://www.rdrop.com/users/paulmck/RCU/linuxusage.html">http://www.rdrop.com/users/paulmck/RCU/linuxusage.html</a>
   <dt id="biblio-stdconsume">[StdConsume]
   <dd>JF Bastien. <a href="https://github.com/jfbastien/stdconsume">GitHub: jfbastien / stdconsume</a>. October 9, 2017. URL: <a href="https://github.com/jfbastien/stdconsume">https://github.com/jfbastien/stdconsume</a>
  </dl>